Interferometric measuring devices having a modulation interferometer and a downstream reference interferometer are used for optical distance measurement, for example, in quality assurance when measuring surface geometries. German Patent No. DE 198 08 273 describes such an interferometric measuring device for detecting the shape or distance in particular of rough surfaces, having at least one spatially coherent beam generating unit whose radiation is split, in a measuring probe, into a reference measuring beam conducted through a reference measuring branch and reflected therein, and a measuring beam conducted through a measuring branch and reflected on the rough surface, having a device for modulating the light phase or for shifting the light frequency (heterodyne frequency) of a first partial beam with respect to the light phase or the light frequency of a second partial beam, having a superimposition unit for superimposing the reflected measuring reference beam on the reflected measuring beam, having a beam decomposition and beam receiving unit for splitting the superimposed radiation into at least two beams having different wavelengths and for converting the radiation into electric signals, and having an analyzer unit in which the shape and/or distance of the rough surface is determinable on the basis of a phase difference of the electric signals. The radiation emitted by the beam generating unit is short-coherent in time and broad-band.
Such interferometric measuring devices initially composed of two interferometers may be constructed using different interferometer types. The modulation interferometer may thus be designed as a Mach-Zehnder interferometer, while the measuring interferometer or the measuring probe may be designed as a compact interferometer, for example, a Mirau interferometer. The interferometric measuring devices share the feature that the path difference between two partial beams, established in the first interferometer, is equalized again by a short-coherent radiations source in the second measuring interferometer or measuring probe and the partial beams are thus brought to interference. The path difference established by a delay element in German Patent No. DE 198 08 273 may also be produced by partial arms of different lengths, through which the partial beams pass, as in a modulation interferometer having optical fibers described in German Patent No. DE 198 08 273.
In order to improve the measuring accuracy of the interferometric measuring device, it is known to connect a reference interferometer to a second output of the modulation interferometer. The reference interferometer has an optical design like that of the measuring interferometer, i.e., it again equalizes the path difference between the two partial beams established in the modulation interferometer. However, the design of the reference interferometer differs from that of the measuring interferometer. The measuring accuracy of the interferometric measuring device may be improved by comparing the signals of the reference interferometer with those of the measuring interferometer.
The path difference to be established in the modulation interferometer depends on the design of the measuring interferometer or the measuring probe. After a replacement of the measuring interferometer/measuring probe, the path difference in the modulation interferometer must be adjusted. This is usually accomplished motively by shifting optical components.
The path difference must be adjusted accordingly also in the reference interferometer. In this case the path difference is typically adjusted by replacement of a pre-adjusted unit. The disadvantage here is that units adjusted to the measuring interferometer used must be available. The replacement of units is complicated.
An object of the present invention is to provide a device of the above-mentioned type which avoids the aforementioned disadvantages and makes a replacement of measuring probes possible using less adjustment of the reference interferometer.